In the manufacture of pressure-sensitive recording papers, better known as carbonless copy papers, a layer of pressure-rupturable microcapsules containing a solution of colorless dyestuff precursor is normally coated on the back side of the front sheet of paper of a carbonless copy paper set. This coated backside is known as the CB coating. In order to develop an image or copy, the CB coating must be mated with a paper containing a coating of suitable color developer, also known as dyestuff acceptor, on its front. This coated front color developer coating is called the CF coating. The color developer is a material, usually acidic, capable of forming the color of the dyestuff by reaction with the dyestuff precursor. Marking of the pressure-sensitive recording papers is effected by rupturing the capsules in the CB coating by means of pressure to cause the dyestuff precursor solution to be exuded onto the front of the mated sheet below it. The colorless or slightly colored dyestuff, or dyestuff precursor, then reacts with the color developer in the areas at which pressure was applied, thereby effecting the colored marking. Such mechanism or the producing technique of pressure-sensitive recording papers is well known.
Among the well-known basic, reactive, colorless, chromogenic dye precursors conventionally used in such carbonless copy paper systems, include those belonging to the classes of the phthalides, fluoranes, spiropyranes, azomethines, triarylmethane-leuco dyes, of the substituted phenoxazines or phenothiazines, and of the chromeno or chromane color formers. Examples of such suitable color precursors are: crystal violet lactone, 3,3-(bisamino-phenyl)-phthalides, 3,3-(bisubstituted indolyl)-phthalides, 3-(aminophenyl)-3-indolyl-phthalides, 6-dialkylamino-2-n-octylaminofluoranes, 6-dialkylamino-2-arylaminofluoranes, 6-dialkylamino-3-methyl-2-arylaminofluoranes, 6-dialkylamino-2- or 3-lower alkylfluoranes, 6-dialkylamino-2-dibenzylaminofluoranes, 6-dialkylamino-2-dibenzylaminofluoranes, 6-diethylamino-1,3-dimethylfluoranes, the lactonexanthenes, the leucoauramines, the 2-(omega substituted vinylene)-3,3-disubstituted-3-1-1-indoles, 1,3,3-trialkylindolinospirans, bis-(aminophenyl)-furyl-, phenyl- or carbazolylmethanes, or benzoyl-leucomethylene blue.
Known color developers for use in such recording papers have included:
(1) novolac phenolic resins made by acid catalyzed condensation of phenol, resorcinol, pyrogallol, cresols, xylenols, or alkyl phenols, such as p-tertiary butyl phenol, with aldehydes such as formaldehyde, acetaldehyde, benzaldehyde and butyraldehyde; PA0 (2) metal salts of aromatic carboxylic acids with an OH group at the ortho position, such as zinc salts of salicylic acid, 3,5-di-tert-butyl salicylic acid, octyl salicylic acid, and 1-hydroxy-2-naphthoic acid, and PA0 (3) acid-treated clays such as kaolinites and attapulgites.
One of the disadvantages of the use of traditional phenolic resins, such as novoloac-type resins, including the zinc salts of such resins and halogen-substituted resins, is the characteristic of yellowing during storage. Thus, the search has continued for other developers, particularly those having the advantages of phenolic-type resins, having high developing power, rapid developing speed, good light resistance, and time stability without yellowing. Another disadvantage of common phenolic developers is the adverse affect thereon by solvents or solvent vapors.
The compound 4,4'-sulfonyl diphenyl (hereinafter SDP) has been listed as a possible color developer in, for example, U.S. Pat. Nos. 3,244,550, and 3,560,229, all to Farnham et al. Sulfonyl diphenol is also mentioned in U.S. Pat. Nos. 4,264,365 and 4,203,619 to Sanders. Hayashi et al, U.S. Pat. No. 3,834,929 disclose metal phenolates in which the phenol compound used to form the metal phenolate may be 4,4'-dihydroxy-3,3'-dichloro (or 3,3',5,5'-tetrachloro)diphenyl sulfone. Furthermore, Yamaguchi et al, U.S. Pat. No. 4,260,179, disclose 2,2'-bisphenolsulfone zinc salts. The use of 4,4'-dihydroxy-diphenyl sulfone as a color developer in CF coating systems has not achieved commercial acceptance, however, as, while SDP will modestly react with most leuco dyestuffs if properly handled, its sensitivity is affected by deposition conditions, and it is not easily adaptable as a single-component CF reactant.